Solar power generators are categorized roughly into a non-concentrator photovoltaic device which emits sunlight to a solar battery cell as it is and a concentrator photovoltaic device which emits sunlight, which is concentrated by an optical concentrator, to a solar battery cell. In the case of the concentrator photovoltaic device, a solar battery cell can be made small. Accordingly, even if an expensive cell with good conversion efficiency is used, the effect on the cost of manufacturing electric power is slight. Therefore, the concentrator photovoltaic device is advantageous in that inexpensive electric power can be efficiently generated.
Light concentrated by the optical concentrator has a high intensity at its centre and a low intensity at its periphery. Even if such light is directly emitted to a solar battery cell, it is not possible to obtain high power generation efficiency. Therefore, in the concentrator photovoltaic device, a columnar or trapezoidal (tapered) optical member called a homogenizer is usually provided immediately above the solar battery cell. The homogenizer serves to homogenize light energy by repeatedly performing total reflection of high-energy sunlight, which is concentrated by the optical concentrator, on the side surface. Generally, glass with high light transmittance is used for the homogenizer. In particular, sodium containing glass, such as borosilicate glass and silicate glass, is used for the homogenizer since it is an all-purpose and cheap material and can be easily processed.
In addition, a solar battery cell is easily deteriorated by moisture. For example, group III-V compound semiconductor represented as InGaP/InGaAs/Ge is active compared with a crystalline silicon based semiconductor. Therefore, a solar battery cell using the group III-V compound semiconductor is noticeably deteriorated by moisture. In addition, an antireflection film is generally provided on the surface of a solar battery cell. However, the antireflection film may be altered if the antireflection film comes in contact with moisture. In addition, the sodium contained in the homogenizer may dissolve in condensed water and sodium ions may reach the solar battery cell through moisture. The sodium ions which reach the solar battery cell have accumulated a negative electric potential of the surface of the solar battery cell, resulting in a reduction of power generation efficiency. Therefore, in order to improve the durability of the concentrator photovoltaic device, it is necessary to protect the solar battery cell against moisture.
Various proposals have been made regarding a method of protecting such a solar battery cell from moisture.
For example, Patent Document 1 discloses a concentrating type solar power generating unit that uses a material containing 10% by weight or more of fluorinated silicon resin as a sealing resin (sealant) which covers the columnar optical member (homogenizer) and the solar battery cell facing the bottom surface.
Patent Document 1 also discloses the following points.
(a) If a material containing 10% by weight or more of fluorinated silicon resin is used as a sealing resin, permeation of vapor is suppressed due to the low vapor transmission property of the fluorinated silicon resin.
(b) A thin film, which is formed of fluororesin (refractive index: 1.34) with a thickness of about 10 nm to 20 nm and which functions as a protective member or a water repellent film, may be formed on the side surface of the homogenizer.
In addition, Patent Document 2 discloses a concentrator photovoltaic device in which transparent resin is provided between the bottom surface of the columnar optical member and the solar battery cell and which includes a light shielding member for blocking sunlight from the transparent resin.
Patent Document 2 also discloses the following points.
(a) Since photodegradation of the transparent resin is suppressed by the light shielding member, deterioration of the solar battery caused by the permeation of moisture is suppressed.
(b) A thin film, which is formed of fluororesin (refractive index: 1.34) with a thickness of about 10 nm to 20 nm and which functions as a protective member or a water repellent film, may be formed on the side surface of the homogenizer.
Since the sealing resin or the transparent resin for protecting a solar battery cell is exposed to the severe environment of the concentrator photovoltaic device, heat resistance and weather resistance are necessary. Materials containing silicon resin as a base material are currently used for these purposes. Generally, silicon resin has good weather resistance. However, since it is used in an area in contact with a severe environment, it is not possible to ensure sufficient weather resistance just through silicon resin. Therefore, a material obtained by adding a filler (for example, a glass compound) for increasing the weather resistance of silicon resin is generally used as the sealing resin.
The refractive index of silicon resin containing glass compound is about 1.5, which is close to the refractive index (about 1.6) of a homogenizer. If the periphery of a homogenizer is covered with sealant formed of a material with such a relatively high refractive index, the critical angle of the total reflection of light of the portion becomes larger than the critical angle of a portion which is not covered with the sealant.
On the other hand, when the shape of the homogenizer is a trapezoidal shape with a smaller sectional area at the solar battery cell side, the incidence angle of light (the angle between the normal line direction of the reflection surface and the light incidence direction) becomes small whenever reflection is repeated. Accordingly, if the lower side surface of the homogenizer is sealed with a high refractive index material, the probability that the incidence angle will become equal to or smaller than the critical angle (that is, the probability that light will leak) is higher near the lower side surface of the homogenizer.
In order to solve this problem, using a material with a relatively low refractive index as the sealant may be considered. However, there is no known material which has a low refractive index and is excellent in heat resistance and weather resistance.